Regarding the background of the invention it should be noted that on measuring the oxygen concentration in process fluids with electrochemical sensors it is necessary to perform a regular check of the functionability of the sensor and the entire measuring device. In particular, with inertisation measurements, in which the oxygen concentrations have to be kept below a maximum level, in order e.g. to avoid ignition or explosion, a regular monitoring of the measuring device is mandatory. An added complication is that electrochemical sensors with a low oxygen concentration only supply a very low measurement current and it is thus difficult to distinguish whether the sensor is defective—e.g. a connection cable is interrupted—or whether in fact the oxygen concentration in the process fluid is very low.
It has previously been usual when checking the function to charge the sensor regularly with a testing gas, such as e.g. air. In addition, it has been necessary to remove the sensor or to use very expensive manual or automatic devices to separate the sensor from the measuring medium and charge it with testing gas.
In this connection it is known from EP 0 744 620 B1 or DE 10 2005 028 246 B4 to equip electrochemical gas sensor arrangements with gas generators, which are operated for checking the sensor itself and produce a testing gas—generally oxygen—in situ, whereby the sensor can be charged with the testing gas and its function can be checked.
The disadvantage of these known gas sensor arrangements is the fact that specifically for the production of testing gas a gas generator has to be provided, which means additional expense for apparatus.
Known electrochemical sensors for measuring the oxygen partial pressure comprise:                an electrolyte-filled sensor body, which is covered on one side charged with the process fluid by an oxygen-permeable membrane,        a cathode on the membrane,        an annular guard electrode surrounding the cathode, which in measuring operation lies at the same potential as the cathode,        an anode charged by the electrolyte in the sensor body, and        a reference electrode charged by the electrolyte in the sensor body, wherein between the anode and cathode a voltage can be applied, which can be controlled between the cathode and reference electrode at a constant polarization voltage (potentiostat) and the measuring sensor current flowing in measuring operation between the cathode and anode is a measure for the oxygen partial pressure in the process fluid.        
Such sensors usually consist of a three-electrode-arrangement, the mentioned cathode, anode and reference electrode. The purpose of the anode is to keep the potential of the cathode relative to the reference electrode at a stable value, the so-called polarization voltage. The reduction of the oxygen at the cathode causes a current flow from the cathode to the anode which is proportional to the oxygen partial pressure. The whole system is covered by an oxygen-permeable membrane, in order to prevent the exchange of the electrolyte with the measuring medium and other impurities.
When measuring lower oxygen concentrations usually a sensor is used in which the cathode is surrounded by an additional so-called guard ring—this is the guard electrode—, which is at the same potential as the cathode and reduces the remaining oxygen.